1. Field of the Invention
This invention relates generally to wireless communication systems. More particularly, it relates to a wireless communication system using a plurality of antenna elements with weighting and combining techniques for optimizing antenna diversity and combining gain.
2. Description of the Related Art
Recently, the market for wireless communications has enjoyed tremendous growth. Wireless technology now reaches or is capable of reaching virtually every location on the face of the earth. Hundreds of millions of people exchange information every day using pagers, cellular telephones and other wireless communication products.
With the appearance of inexpensive, high-performance products based on the IEEE 802.11a/b/g Wireless Fidelity (Wi-Fi) standard, acceptance of wireless local area networks (WLANs) for home, Small Office Home Office (SOHO) and enterprise applications has increased significantly. IEEE 802.11b/g is a standard for a wireless, radio-based system. It operates in the unlicensed 2.4 GHz band at speeds up to 11M bits/sec for IEEE 802.11b and 54 M bits/sec for IEEE 802.11g. The IEEE 802.11b/g specification sets up 11 channels within the 2.4 GHz to 2.4835 GHz frequency band which is the unlicensed band for industrial, scientific and medical (ISM) applications. IEEE 802.11a is another standard for a wireless, radio-based system in the ISM band. It operates in the unlicensed 5-GHz band at speeds up to 54 M bits/sec.
It has been found that WLANs often fall short of the expected operating range when actually deployed. For example, although a wireless Access Point (AP) is specified by a vendor as having an operating range of 300 feet, the actual operating range can vary widely depending on the operating environment.
In particular, WLAN performance can be greatly degraded by direct and multipath radio interference. Multipath occurs in wireless environments because the radio frequency (RF) signal transmitted by the subscriber is reflected from physical objects present in the environment such as buildings. As a result, it undergoes multiple reflections, refractions, diffusions and attenuations. The base station receives a sum of the distorted versions of the signal (collectively called multipath).
Similarly, in any indoor wireless system, multipath interference effects occur when the transmitted signal is reflected from objects such as walls, furniture, and other indoor objects. As a result of multipath, the signal can have multiple copies of itself, all of which arrive at the receiver at different moments in time. Thus, from the receiver's point of view, it receives multiple copies of the same signal with many different signal strengths or powers and propagation delays. The resultant combined signal can have significant fluctuation in power. This phenomenon is called fading.
There are additional elements of performance degradation in a network of 802.11b/g WLAN access points (APs). Since the 802.11b/g channel bandwidth is approximately 16 MHz, only three non-overlapping channels operating in proximity can be accommodated without interfering with one another. The channel re-use factor imposes a severe restriction on implementation of 802.11b/g based systems which requires significantly more effort in the network deployment, and increases the chances of interference and packet collision especially within an environment with a dense user cluster, such as in an office building or apartment building. It is not usual that a user can see more than 10 different APs simultaneously. Multipath interference further complicates the situation because being physically closer to an AP does not mean the signal from the AP is stronger. Signal propagates from a different path from a remote AP can have stronger power. Thus, site survey to determine the signal propagation is often required for a corporation trying to deploy multiple APs within an office complex.
Several approaches for improving the operating performance and range in a fading environment have been suggested. In one conventional approach, selection antenna diversity is used to reduce the effect of multipath fading. Multiple antennas are located in different locations or employ different polarizations. As long as the antennas have adequate separation in space or have a different polarization, the signal arriving at different antennas experiences independent fading. Each antenna can have a dedicated receiver or multiple antennas can share the same receiver. The receiver(s) checks to see which antenna has the best receiving signal quality and uses that antenna for the signal reception. The performance gain thus achieved is called diversity gain. The performance gain increases with the number of diversity antennas. The drawback of the selection diversity approach using a single shared receiver is that fast antenna switching and signal quality comparison is required. Since an 802.11(a, b, g) signal has a short signal preamble, only two diversity antennas are typically employed. This achieves a diversity gain of approximately 6 dB in a flat Rayleigh fading environment at the required frame error rate. The diversity gain decreases to 3 dB when delay spread is 50 ns and 0 dB when delay spread is 100 ns.
In another conventional approach, signal combining is used to provide improved performance in a fading environment. Signal combining techniques employ multiple spatially separated and/or orthogonally polarized antennas. The received signal is obtained by combining the signals from the multiple antennas. One technique for providing optimal signal quality is known as maximal ratio combining (MRC). To achieve the best signal quality, the received signal from each antenna is phase-shifted such that the resultant signals from all antennas are in phase. In addition, the signal from each antenna is scaled in amplitude based on the square root of its received signal-to-noise ratio.
Another known approach to achieve performance improvement is through equalization, either in the time or frequency domain. In this technique, the multipaths arriving at the receiver are delayed, phase shifted, and amplitude scaled before they are combined (equalized). Equalization typically works better when the delay spread is large (>100 ns). The performance enhancement as a result of equalization adds to the diversity gain of antennas.
In U.S. patent application Ser. No. 10/732,003, filed Dec. 10, 2003 entitled Wireless Communication System Using a Plurality of Antenna Elements with Adaptive Weighting and Combining Techniques, a closed loop operation system which can simultaneously perform signal combining using MRC and adjacent channel interference suppression using INA are proposed. The proposed approach can be unstable in some cases and unable to achieve fast convergence and integrator overflow.
Interference suppression and range enhancement approaches are most effective if the solutions fit in a PCMCIA form factor. Solutions typically are implemented with ASIC (application specific integrated circuit) to reduce the required space. A WLAN typically employs two spatially separated omni-directional antennas to have better coverage and each antenna typically requires approximately a quarter wavelength in the resonant dimension. A solution that requires more than two antenna elements needs to fit within the PCMCIA form factor so as not to limit the available combining and diversity gain.
Additional requirements for WLAN performance enhancement are low power consumption, minimal implementation cost, and high reliability. Since WLAN client devices are typically installed in battery powered notebook computers, a low power and low cost solution are criteria for the success of a commercial product. The use of digital signal processing techniques for any solution involving multiple antenna elements uses high power and has high costs. Since each antenna element requires two high-speed analog to digital converters (ADC), a solution involving four antenna elements would use eight high speed ADCs, thereby having higher cost and higher power consumption.
Alternatively, analog implementations are typically complicated by poor component tolerances and high IC process and temperature-dependent parameter variations.
It is desirable to provide an enhanced communication system to provide diversity, combining gain or interference suppression techniques which can be self-aligned and converges to the correct parameter values, independent of process, temperature, and component variations.